Test cross compilation with `proc-macro` crates

[jseyfried]

In particular, test #37899.

[est31]

Can you actually somehow distinguish between target and host on proc-macro crates? What if you want to implement a feature for one given target architecture, independent from whether it gets created by cross compilation or normal compilation? How do build.rs scripts handle this?

[jseyfried]

@est31 I think we shouldn't support cross compiling proc-macro crates -- that is, we always compile proc-macro crates for the host architecture. This issue is calling for testing of cross compiled crates that depend on proc-macro crates (which are compiled for the host) as well as other dependencies (which are compiled for the target).

[emk]

I've posted a stripped-down example program which triggers this error.

[Mark-Simulacrum]

This seems like a E-needstest, probably sufficient to just have a run-make test I guess, which will then be tested on the cross test platforms. @jseyfried Does that sound correct? Please untag as needstest if that's wrong.

[jseyfried]

@Mark-Simulacrum Yeah, that sounds correct -- a run-make test is probably best here.

[steveklabnik]

Triage: not aware of anything here changing, but I'm also not an expert.